This invention relates to systems and methods of monitoring thin film deposition.
Monitoring and controlling the deposition of thin films, for example, by vapor deposition or sputtering, are required steps in the production of high quality thin film devices. Quartz crystal deposition monitors typically are used to monitor the deposition process and to control the amount of material deposited on a substrate and the rate at which material is deposited on the substrate. In practice, a quartz crystal monitor is mounted on a water-cooled holder that is positioned inside a vacuum deposition chamber where material may deposit on the exposed monitor surface while a thin film is deposited on a substrate that is located near the exposed monitor surface. Due to the large size of the monitor crystal and the associated holder, the monitor often is mounted at a location that is offset from the substrate. As a result, the monitor typically is exposed to different deposition conditions than the substrate. This difference often is corrected by a tooling factor.
In general, as material is deposited onto the exposed surface of a quartz crystal monitor, the resonant frequency of the monitor decreases. Quartz crystal is characterized by a relatively high quality factor (Q), which enables quartz crystal monitors to make high resolution frequency measurements and, thereby, allows such monitors to detect small changes in the deposited mass. For example, a monolayer of copper added to a 6 MHz quartz crystal monitor reduces the resonance frequency by approximately 20 Hz, which is on the order of the bandwidth of the resonator. To a first order approximation, the gauge factor (MHz/nm) of a quartz crystal monitor is proportional to the density of the deposited material. Typical thin film monitor quartz crystals have quality factors that are on the order of 200,000 and, consequently, are characterized by a 3 dB line width and a frequency shift resolution that are on the order of 20 Hz. Typical commercial instruments (e.g., a quartz crystal thin film deposition monitor available from Inficon of East Syracuse, N.Y. U.S.A.) have resolutions that are on the order of 0.1-0.2 nm for film thicknesses in the range of 100 nm, or less. The absolute thickness accuracy of such systems is on the order of 1-2%.
In a typical instrumentation arrangement, the resonant frequency of a quartz crystal monitor is determined by placing the monitor in the feedback loop of an external oscillator circuit as a frequency control element. The quartz crystal monitor is connected to the oscillator circuit by a cable that extends through a vacuum feed-through in a wall of the deposition chamber. Since the equivalent electrical impedance of the quartz crystal is pure real and maximized at its parallel resonance frequency and is minimized at its series resonance frequency, the oscillator circuit tends to produce an output signal at one of the other of the crystal resonance frequencies, as determined by the external oscillator circuit. As a result, changes in the crystal resonance frequency produce corresponding changes in the oscillator circuit output frequency, which may be monitored by an external control circuit.
The invention provides a novel scheme (systems and methods) for monitoring thin film thickness or substrate temperature, or both.
In one aspect, the invention features a system for monitoring a thin film deposition that includes a thin film deposition sensor comprising an acoustical resonator that has an exposed surface exposed and is responsive to thin film material deposits on the exposed surface.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
The acoustical resonator preferably is a thin film bulk acoustical resonator (FBAR). The thin film deposition sensor preferably further comprises a second acoustical resonator thermally coupled to the first acoustical resonator and shielded from deposition of thin film material. The first and second acoustical resonators may be coupled electrically in series or in parallel, or may be addressed individually. The first and second acoustical resonators may be coupled together by an electrical connection that is shielded from thin film material deposits. In one embodiment, a plurality of pairs of exposed and shielded acoustical resonators are disposed on an elongated substrate.
The monitoring system preferably includes an antenna configured to enable the thin film deposition sensor to be interrogated wirelessly. Alternatively, an optical coupler may be used to interrogate the thin film deposition sensor wirelessly.
In another aspect, the invention features a thin film deposition monitoring system that includes a thin film deposition sensor and a substrate clip that is configured to attach the thin film deposition sensor to the substrate. In this way, the monitor may be exposed to substantially the same deposition conditions as the substrate, in which case convention tooling factor corrections may not be needed.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
The substrate clip may include an antenna.
In another aspect of the invention, a thin film deposition monitoring system includes a thin film deposition sensor and a transceiver circuit that is configured to enable the thin film deposition sensor to be interrogated wirelessly.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
In some embodiments, the monitoring system may include a first antenna coupled to the thin film deposition sensor and a second antenna coupled to the transceiver circuit.
The transceiver circuit may include an energy storage element and an opto-electronic transducer.
In one embodiment, the transceiver is an RFID tag circuit.
In another aspect, the invention features a method of monitoring a thin film deposition on a substrate. In accordance with this inventive method, a thin film deposition sensor comprising an acoustical resonator is disposed within a deposition chamber, and a surface of the acoustical resonator is exposed to a thin film deposition.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
The thin film deposition sensor preferably is interrogated wirelessly to determine the resonant frequency of the acoustical resonator. The thin film deposition sensor preferably further comprises a second acoustical resonator that is shielded from the thin film deposition. The thin film deposition sensor preferably is interrogated wirelessly to determine the resonant frequencies of the first and second acoustical resonators.
In some embodiments, an optical signal may be transmitted through an optical port of the deposition chamber.
The thin film deposition sensor may be disposed within the deposition chamber by attaching the thin film deposition sensor to the substrate.
Among the advantages of the invention are the following.
The invention provides a novel thin film deposition monitoring system that may be used to monitor thin film thickness or substrate temperature, or both. The use of a pair of exposed and shielded acoustical resonators, which respond to temperature changes in substantially the same way, enables a controller to distinguish temperature-induced changes in resonant frequency from mass-induced changes in resonant frequency. This feature avoids the need to maintain the thin film deposition sensor at a constant controlled temperature and, thereby, avoids the need for a water-cooled holder and associated cooling equipment (e.g., water pipes).
In addition, the novel substrate clip provides a convenient way to implement a thin film deposition sensor as a single-use, disposable thin film thickness and temperature monitor. This feature avoids the need to periodically replace monitors and the associated risk that effluent build-up might flake off and contaminate the vacuum deposition system. Also, because the thin film deposition sensor may be clipped directly to the substrate, the invention allows a controller to monitor the film growth at the substrate surface. This feature avoids inaccuracies that could result from monitoring deposition conditions at a location displaced from the substrate, conditions which may not correlate well with the actual deposition conditions at the substrate surface. Thus, conventional tooling factor corrections may be eliminated for the most part.
Furthermore, because the novel acoustical resonators are characterized by relatively small dimensions, multiple redundant thin film deposition sensors may be disposed within a vacuum chamber to provide a plurality of data points that enable a controller to monitor and control the deposition process with greater accuracy. In this way, the deposition uniformity at the substrate surface may be monitored and controlled dynamically (e.g., in vacuum deposition systems that include multiple independently controllable material sources).